(a) Field
The present invention relates to a LED emitting device and a driving method thereof. More particularly, the present invention relates to an LED emitting device and a driving method thereof controlling an overvoltage generated when driving the LED.
(b) Description of the Related Art
A light emitting device (hereafter, an LED emitting device) using LEDs supplies currents to the LEDs so as to drive them. Then, the LEDs emit beams having brightness corresponding to the currents. Such an LED light emitting device can be used as a light source for an LCD or for lighting.
An LED light emitting device emits light having brightness by making a predetermined current flow in each LED channel including a plurality of LEDs connected in series in a voltage application state of more than a threshold voltage. The threshold voltage is a minimum voltage required to turn on a plurality of LEDs forming the LED channel.
The operation of emitting the light by supplying the current to the LED channel is referred to as a turn-on, and the operation stopping the light emitting by blocking the current supply to the LED channel is referred to as a turn-off.
The LED emitting device includes a plurality of LED channels, and constantly controls a current flowing in each of the plurality of LED channels
The plurality of LED channels are coupled in parallel such that power source voltages respectively applied to the plurality of LED channels are the same. The power source voltage is increased/decreased according to increasing/decreasing the number of turned on LED channels among the plurality of LED channels. That is, if the number of turned on LED channels is increased, the power source voltage is decreased, and if the number of the turned on LED channels is decreased, the power source voltage is increased.
Thus, the LED emitting device detects the increasing/decreasing of the power source voltage to constantly control the power source voltage. Generally, the LED emitting device constantly controls the power source voltage with reference to the lowest voltage among the channel voltages. The channel voltage as a voltage of an end of the LED channel means the voltage of which the voltage of both terminals of the LED channel is subtracted from the power source voltage. The power source voltage must be controlled with reference to the lowest channel voltage among the plurality of LED channel voltages, and thereby a plurality of LEDs of all LED channels may be turned on by the power source voltage.
That is, the LED channel having the lowest channel voltage means that the voltage between both terminals of the LED channel is highest. If the power source voltage is less than the highest voltage among the voltages between both terminals generated when the constant current flows in the LED channel, the power source voltage is less than the threshold voltage such that current required for the LED channel to emit light may not be supplied. Accordingly, the power source voltage is controlled with reference to the lowest channel voltage of the plurality of LED channel voltages.
The lowest channel voltage of the plurality of LED channel voltages is maintained to the time that the channel voltage is sampled in the LED emitting device and the further lower LED channel voltage is generated. When the number of turned on LED channels is decreased, the power source voltage is increased, and the voltage that is less than the sampled LED channel voltage is not generated. If the sampled LED channel voltage is maintained in this state, an overvoltage phenomenon of the power source voltage is generated.
To prevent this, if the power source voltage is increased to a predetermined overflow reference voltage, the LED emitting device controls the power source voltage such that the power source voltage is decreased to a predetermined stabilization voltage.
During a period from the time that the power source voltage is increased to the overflow reference voltage to the time that it is decreased to the stabilization voltage, the number of turned on LED channels among the plurality of LED channels may be increased. Thus, this period is a period in which the power source voltage is decreased such that it may be difficult to supply the constant current to the LED channel. That is, the LED channel that is not turned on is generated before the power source voltage is increased to the appropriate voltage to supply the constant current to all LED channels that must be turned on.
As described above, when the overvoltage of the power source voltage is generated such that the number of LED channels that must be turned on is increased during the period in which the power source voltage is stabilized, a serious problem that the turn-on time of the LED channel is delayed is generated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.